<!DOCTYPE html>
<style>
#scroll-container {
  width: 400px;
  height: 400px;
  overflow: scroll;
}

#scroll-contents {
  width: 1000px;
  height: 1000px;
  position: relative;
}

#anchor {
  anchor-name: --anchor;
  height: 100px;
  width: 100px;
  overflow: scroll;
}

#anchored {
  background: green;
  width: 100px;
  height: 100px;
}
</style>

<div id="scroll-container">
  <div id="scroll-contents">
    <div style="height: 400px"></div>
    <div id="anchored"></div>
    <div id="anchor">
      <div style="height: 500px"></div>
    </div>
  </div>
</div>

<script>
document.getElementById('scroll-container').scrollTop = 300;
document.getElementById('anchor').scrollTop = 300;
</script>
